The present invention relates to fuel injectors and more particularly to coatings or surface treatments for fuel injector components such as fuel injector plungers.
Many internal combustion engines, whether compression ignition or spark ignition engines, are provided with fuel injection systems to satisfy the need for precise and reliable fuel delivery into the combustion chamber of the engine. Such precision and reliability is necessary to address the goals of increasing fuel efficiency, maximizing power output, and controlling undesirable by-products of combustion.
A fuel injector is a precision device that must meter the quantity of fuel required for each cycle of the engine and must develop the high pressure necessary to inject the fuel into the combustion chamber at the correct instant of the operating cycle. Many fuel injection units utilize a hydraulically actuated mechanism to pressurize the fuel charge to obtain the desired fuel spray pattern. The hydraulically actuated system operates on a fuel injection plunger that is disposed within a bore formed in the fuel injector for engaging a nearly incompressible liquid fuel. This mechanical pressurization of the liquid fuel produces an extremely high fuel injection pressure, often exceeding 140,000 MPa(20,000 psi).
Additionally, the hydraulically actuated electronically controlled unit injector often operates at a more severe condition than the mechanically actuated electronically controlled unit injector. The hydraulically actuated electronically controlled unit has a shorter fuel injector plunger and thus, a shorter engaged length in the mating barrel. Experimentation has shown the length of the hydraulically actuated electronically controlled unit injector""s plunger engagement is about one-half of the mechanically actuated electronically controlled unit injector plunger.
The decreased surface area in which to spread out the contact stress causes the hydraulically actuated electronically controlled unit injector to experience increased wear at the contact surfaces relative to mechanically actuated electronically controlled unit injectors, which have considerably longer engaged length between the fuel injector plunger and the mating barrel. The hydraulically actuated electronically controlled unit injector requires a superior lubrication boundary layer to combat the higher contact stress.
The operability of a fuel injector is dependent, to some extent, on the fuel to be injected, and in particular on the lubricity, viscosity or other salient physical characteristics of the fuel to be injected. The use of low lubricity fuels, in particular, can cause several problems, most notably fuel injector plunger scuffing, and ultimately the seizure of the fuel injector plunger within the bore of the fuel injector. Scuffing is typically caused by lack of lubrication at the interface between two hard surfaces causing a welding or adhesion of the contacting parts, e.g. fuel injector plunger and fuel injector barrel sliding against one another without proper lubrication tend to show evidence of scuffing. Scuffing, which is a form of wear, will change the clearance between the plunger and the barrel and will make the surface rough so the sliding motion of the plunger will not be smooth, both of which will lead to an incorrect amount of fuel injected into the system. Eventually, continued scuffing can lead to seizure of the fuel injector plunger in the barrel as the plunger and barrel will effectively merge into one part with no movement allowed. As indicated above, scuffing of fuel injector components is particularly evident in fuel injection systems that utilize low lubricity fuels.
The aforementioned problems exist primarily, although not exclusively with low lubricity fuels and other non-standard fuels including gasoline, naphtha, D1 type diesel fuels, aviation fuels, blended or heavy fuels, crude oils or water/fuel emulsions, including water continuous and fuel continuous emulsions.
Various related art techniques have considered the use of titanium nitride (TiN) coatings on fuel injector plungers to reduce wear of the coated parts. The problem encountered with TiN coatings is that a TiN coating is usually applied at extremely high temperatures (e.g. about 450 degrees C.) which may produce unwanted thermal stresses and related failures to the fuel injector plunger. It is also believed that TiN coatings on fuel injector plungers tend to increase the wear of the plunger mating component at the mating or seating location. In addition, TiN coatings tend to increase the overall cost of the fuel injector plunger because the TiN coated fuel injector plunger requires tool grade steel to withstand the high temperatures observed in the application of the TiN coating.
Alternatively, several related art techniques have considered the use of ceramic materials as the base material for the fuel injector plunger instead of metal. Unfortunately, the use of a ceramic material for fuel injector plungers is very costly and the resulting monolithic ceramic fuel injector plungers have not typically demonstrated the necessary durability or reliability for commercial use in diesel engines or other heavy duty engine applications.
The present invention aids in overcoming one or more of the aforementioned fuel injector problems and the shortcomings of the related art solutions to such problems.
The invention may be characterized as a diamond like carbon coated low alloy steel component for a fuel injection system having improved boundary lubrication characteristics, and improved scuffing resistance characteristics are disclosed. The component comprises a steel substrate and a primary coating (e.g. tungsten carbide containing carbon) deposited on the substrate. The primary coating has a thickness generally no greater than about 5 microns.
In one aspect of the present invention, a process for increasing the boundary lubrication of low alloy steel lubricated contacts in a fuel injection system is disclosed. The process comprises the steps of preparing a low alloy steel substrate and then forming a metal nitride coating or diamond like carbon coating on the low alloy steel substrate using a low temperature physical vapor deposition process.
A further aspect of the invention is that the disclosed coatings or surface treatments of the fuel system component operate to augment the lubricity of certain low lubricity fuels. The disclosed coatings applied to the fuel injector plunger and the corresponding mating component (e.g. fuel injector barrel) can significantly reduce the possibility of lubricity related failures of the fuel injector. Utilization of such coatings on fuel injector system components can reduce or eliminate the need for adding lubricants to the low lubricity fuel compositions.